Which of the following is a triangular number?
- A.20
- B.21 ✓
- C.22
- D.24
Explanation: Triangular numbers: 1, 3, 6, 10, 15, 21, 28, ... — 21 is the 6th one.
Class 6 IMO Mathematics Olympiad practice · Algebraic Thinking · Patterns in Mathematics
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